This invention relates to an aspherical lens system which is used, for example, in a bar code sensor or reader for sensing a bar code attached to an article or goods.
In general, a bar code sensor of the type described comprises a charge-coupled device (CCD) and an optical lens system for imaging a bar code in a reduced size onto the charge-coupled device. In such optical lens system, an astigmatism, distortion, comae, and the like, must be accurately corrected or adjusted to predetermined ranges.
On the other hand, the charge-coupled device comprises a plurality of charge-coupled elements which are arranged in columns and rows and which are in one-to-one correspondence to single bits to be scanned at every element. Each of the charge-coupled elements often has a size of 14 micron meters. Under the circumstances, it is known that the optical lens system for such a charge-coupled device must have resolving power of 60% for 12 line pairs/mm (l.sub.p /mm).
In order to accomplish the above-mentioned resolving power, a conventional lens system of the type described comprises a spherical lens system of a triplet type composed of three lenses divided into three groups. Such a conventional lens system will be referred to as a triplet type spherical lens system. Alternatively, another conventional lens system comprises a plurality of groups which are greater than three groups and which may be called a multi-group lens system.
For example, such a triplet type lens system has been disclosed in Japanese Unexamined Patent Publication No. Sho 60-22109, namely, 22109/1985, assigned to Ricoh Co., Ltd. and comprises a plurality of lenses which are composed of a meniscus lens, a double convex lens, and a double concave lens. The triplet type lens system is developed to be used as an optical lens system for a facsimile device and may be applicable to an optical lens system of the bar code sensor because the optical lens system for the bar code sensor may not be so strict in optical characteristics as compared with the optical lens system for the facsimile device. Therefore, the triplet type lens system disclosed in the above-referenced publication may be readily modified into the optical lens system for the bar code sensor.
However, the triplet type lens system of the above-mentioned type is disadvantageous in that three kinds of lenses must be accurately manufactured to form such a lens system. In addition, the lenses must be accurately mounted in a lens mount. Thus, the lenses and the lens mount should be machined or processed with a high precision. Moreover, the triplet type lens system one requires to precisely assemble the lenses into the lens mount so that the lenses are aligned with one another along optical axis. Inasmuch as manufacturing processes are objectionably increased so as to manufacture the triplet type lens system, the triplet type lens system is very expensive when applied to the bar code sensor.
Consideration might be made about a spherical lens system which is composed of two lenses divided into two groups, respectively. However, it is difficult with this spherical lens system to sufficiently compensate for spherical aberrations and to widen a field angle.